She suffers from an autoimmune disease, but seven-time grand slam winner Venus Williams refused to blame her health for an early exit at the Australian Open.

The former world No. 1 came into the year's first grand slam in good form after reaching the final of a warm-up event in Auckland in New Zealand, but despite winning the first set against Russia's Ekaterina Makarov, Williams slipped to a 2-6 6-4 6-4 defeat.

Williams, ranked 37th in the world, was diagnosed with Sjogren's syndrome -- which causes joint pain and can deplete energy levels -- in 2011. She hasn't won a grand slam since Wimbledon in 2008.

But the American, who will compete in the doubles event alongside her sister and top seed Serena Williams, refused to look for excuses after her defeat by the 22nd seed Makarova, who will face U.S. world No. 141 Irina Falconi in the second round.

Photos:Feeling the heat: Australian Open fears

Photos:Feeling the heat: Australian Open fears

Unhappy memory – Novak Djokovic contemplates his fate ahead of his retirement with heat exhaustion in a quarterfinal match against Andy Roddick at the Australian Open in 2009.

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Photos:Feeling the heat: Australian Open fears

Cooling treatment – Djokovic used ice-soaked towels in an attempt to cool down in the searing afternoon temperatures in Melbourne.

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Photos:Feeling the heat: Australian Open fears

Constant problem – Spectators wear towels to beat the heat as Tatsuma Ito of Japan played Nicolas Mahut of France on the fourth day of the 2012 tournament in Melbourne.

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"I think that's a factor for any professional athlete, so I don't think I'm any different from anyone else," Williams, who made 56 errors during the match, told reporters when asked if her health was the reason for her defeat.

"The last 12 months I have had issues, but this year I definitely am looking forward to having a good run and feeling well."

Williams will now focus her energies on supporting Serena, who wasted no time in wrapping up a 6-2 6-1 win over Australian 17-year-old Ashleigh Barty.

The biggest upset of the opening day saw sixth seed Petra Kvitova slip to a 6-2 1-6 6-4 reverse against Thailand's world No. 87 Luksika Kumkhum.

It was Kumkhum's first ever match with a top 10 player and her victory means she will at least equal her best grand slam performance to date -- reaching the second round in Melbourne last year.

"I didn't play well," conceded Kvitova, a Wimbledon winner in 2011. "I didn't play my game that I really tried all off season to work on. It was a great off season. I was really excited to be here, feeling good.

"But I think that probably I wanted too much, and then everything just fell down."

Australian Sam Stosur delighted the crowd by chalking up a 6-3 6-4 win over Klara Zakopalova of the Czech Republic.

Seventeenth seed Stosur won the U.S. Open in 2011 but has never been beyond the fourth round of her home grand slam.